This invention relates to apparatus for processing exposed lithographic plates, and more particularly to such apparatus for developing exposed lithographic plates and applying a preservative to the surface of the developed plates.
The invention may be regarded as involving improvements upon prior apparatus for processing exposed lithographic plates such as shown in the co-assigned U.S. Pat. Nos. 3,682,078 and 3,608,464 and 3,782,532, each showing apparatus particularly for processing lithographic plates of the type known in the art as additive plates. These plates have a photosensitive coating on one face thereof, the coating being a diazo compound which is adapted to be imaged by exposure through a film negative to an ultraviolet light source. Areas struck by light become oleophilic and insoluble in most solvents and aqueous solutions. Unexposed areas remain soluble in an acid aqueous solution. The plates are developed by application of a developer which is an oil and water emulsion. The oil phase of the developer generally contains a resin such as an epoxy or vinyl resin. The water phase generally contains gum arabic, a synthetic gum or a starch combined with certain acids. The developer is acid, abrasive and viscous. When it is dispersed over the exposed plate, the acid water phase dissolves the unexposed diazo. For best results, it is necessary to rub the developer over the entire plate surface with light pressure by means of a cellulose sponge or a brush. As the developer is rubbed over the surface, the gum combines with the dissolved diazo to form a desensitizer rendering the non-image areas (the areas not struck by light) hydrophilic. The resin phase is deposited on the exposed image of the plate and hardens by solvent evaporation. The rubbing must be carried out for a sufficient time to deposit a dense uniform resin film so that a visual evaluation of the developing operation may be made, and the resin is pigmented for this purpose. The used material must then be flushed by water from the plate surface and not reused since the solvent balance has changed; the redissolved diazo, having altered the pH value, would cause congealing of the resin phase if returned to the developer supply.
While the prior plate processors shown in said U.S. Pat. Nos. 3,608,464 and 3,782,532 have been generally satisfactory, they involve distribution systems for the developer which are of relatively high initial cost and which involve relatively high-cost maintenance to maintain an even and uniform flow of developer. Thus, they have involved a developer distribution system comprising multiple orifices for delivering the developer, each fed by a pump, or fed from a manifold supplied by a single pump. If too little developer is applied, the plates may be improperly desensitized and insufficient resin may be deposited, causing short press life for the plates and difficulty in trapping the printing ink. If too much developer is applied, solvent may not evaporate sufficiently, and this results in short plate life. Uneven distribution may cause all the above, and wastes developer, and with environmental problems it is desirable to minimize discharge of spent developer. While the plate processor shown in said U.S. Pat. No. 3,682,078 has been quite satisfactory, it has left a need for a simpler and more economical processor.
In other prior plate processors, the plates have been fed forward for processing by feed rolls, including an upper and lower roll immediately downstream from the developer rubbing station. It is desirable continuously to wash this upper roll with water, but difficulty has been encountered in washing it with water sprays as this may result in water traveling back on the upper surface of the plates toward the developer rubbing station and diluting the developer, causing improper and incomplete development.
In the plate processors shown in said U.S. Pat. Nos. 3,682,078 and 3,608,464 and 3,782,532, and in other prior plate processors, a preservative (such as an asphaltum, gum and etch composition sold by the assignee of this invention under its trademark A.G.E.) is applied to the developed plate to seal it. In such apparatus having an upper exit roll engaging the upper face of the plate at the exit end of the processor, there has been a problem of the preservative drying and glazing on this roll. This may undesirably cause ridging of the plate, which in turn causes what is called "gum blinding" in the printing press.
Other problems attendant upon use of plate processors of the class described are waste of developer and preservative, and disposition of spent developer washed off the plates in the processor without causing pollution of the environment.